1. Field of the Invention
The present invention relates to a method for pulling a single crystal of heavy weight by a Czochralski method regardless of a mechanical strength of a seed and a necking portion thereof by mechanically holding a part of a growing crystal during growth.
2. Related Art
A Czochralski method has been conventionally known as a method producing semiconductor material such as silicon, which, for example, as shown in FIG. 2(A), comprises the steps of: contacting a seed crystal 52 held by a seed holder 51 with the surface of a raw material melt 54 contained in a crucible 53; then pulling the seed crystal 52 adjusting its speed, while rotating the seed crystal 52 about its rotational axis, to form a neck portion 55 under the seed crystal 52; and in succession growing a cylindrical body 56 of a single crystal with a larger diameter.
In the Czochralski method, the cylindrical body 56 of a grown crystal under the seed crystal 52 can be produced dislocation-free by forming the neck portion 55. In recent years, however, the weight of a single crystal has been on the continuous increase because of demand for a single crystal with a larger diameter or for the purpose of improvement of production efficiency, in which trend nearly insufficient mechanical strengths of a seed crystal 52 and a neck portion 55 have been often experienced. If the neck portion 55 should be broken to let a growing single crystal to be fallen down, there would be a fear of a serious accident. To cope with such an accident, a method and apparatus for mechanically holding a part of a crystal during crystal growth has been adopted, for example, as shown in FIG. 2(B).
In the apparatus of the figure, a stepped engagement portion 57, which is composed of an expanded-diameter portion and a reduced-diameter portion, is formed between a neck portion 55 and a cylindrical body 56 and the stepped engagement portion 57 is held between lifting jigs 58, 58 to pull a growing single crystal. As such techniques, the following literature has been known:, for example, Laid-open Japanese patent applications 62-288191, 63-252991, 3-285893, 3-295893 and the like. Among them, for example, in the case of Laid-open Japanese patent application 3-285893, a seed crystal is pulled while the stepped engagement portion 57 is formed and when the stepped engagement portion 57 reaches a position where lifting jigs 58, 58 are located, the jigs 58, 58 catch the stepped engagement portion 57 and crystal pulling is continued while being held by the jigs 58, 58.
In the conventional method as described above, the weight of the growing single crystal is effectively sustained but, since the lifting jigs 58, 58 contact with the crystal in their motion along a lateral direction when the crystal is caught by the lifting jigs 58, 58, which is apt to generate a force directed to the crystal in the direction, there arises a problem that the single crystal is swayed and has a chance for oscillation thereof to occur.
In the same case, there arises another problem; when a growing crystal is held by mechanical means, a central axis of rotation in a pulling mechanism of a single crystal, which holds a growing single crystal in an earlier stage, has in practical sense an error, which is small, in alignment with a central axis of rotation of a holding mechanism holding the single crystal in a later stage, though the central axes of rotation are required to be a perfect alignment with each other. In such a situation, when pinching action of the lifting jigs 58, 58 is effected, a plurality of the tips of the jigs 58, 58 do not all touch the crystal with one accord and one of them touches in advance of the others, so that the growing crystal is forced to sway and oscillate.
As described above, it is generally difficult to catch a part of a growing crystal for mechanically holding in a Czochralski method without giving any oscillation to the crystal. And once the growing crystal sways and the oscillation occurs, the crystal is subject to starting to grow with dislocations and to produce bends in the crystal and, as a result, a yield of single crystal is reduced, and, if the oscillation is extreme, shut-down of the pulling process becomes the single choice.
In such circumstances, effective means for suppressing oscillation of a growing crystal has been accordingly desired in a Czochralski method, in a case where part of the growing crystal is mechanically held.